1. Field of the Invention
The present invention relates to the casings for x-ray image intensifier tubes and, more particularly, to the means used in these casings to achieve protection from X-radiation.
2. Discussion of the Background
X-ray image intensifier tubes (in abbreviated form "XII tube" hereafter) are vacuum tubes comprising an input screen toward the front of the tube, an electron optical system, and a screen located at the back of the tube for observing a visible image.
FIG. 1 diagrammatically shows such an XII tube. The tube comprises a glass vacuum enclosure 2 having one end at the front of the tube which is closed by a input window 3 exposed to x-photon radiation.
The second end of enclosure 2 forming the back of the tube is closed by an output window 4 that is transparent to light.
Generated x-rays are converted into light rays by a scintillator screen 5. The light rays excite a photocathode 6 which in response emits electrons. These electrons are accelerated toward output window 4 using different electrodes 7 placed along a longitudinal axis 9 of the tube, and which form the electron optical system. In the example shown, output window 4 carries a cathodoluminescent screen 10, made of luminophores for example. The impact of the electrons on cathodoluminescent screen 10 makes it possible to restore an image formed at the beginning on photocathode 6.
The XII tube is contained in a protective enclosure casing 15, which in particular assures the following functions:
1) protection of electron optical system 7 and of the tube from spurious outside magnetic fields;
2) protection of persons from x-radiation, both direct x-radiation that is not absorbed by scintillator 5, and scattered x-radiation produced by the tube itself;
3) mechanical protection of vacuum enclosure 2 from the outside environment; and
4) to form the fastening interface with radiological equipment (not shown) on which the tube must be mounted, and with accessories for using this equipment, which may comprise, for example, threaded holes for the fastening at faces or planes of reference.
For this purpose, the wall of a standard casing 15 for an XII tube generally comprises several superposed layers 16, 17, 18.
The most outside layer 16 forms a jacket 16 which is the mechanical framework of casing 15. This jacket can be, for example: a cast of metal alloy; or else worked or stamped; or further, mechanical-welded, made from metal foils. Outside layer or jacket 16 assures the mechanical protection of the vacuum enclosure, and particularly in its back part 13, located toward output window 4 of the tube, it performs the fastening interface role. The image produced by cathodoluminescent screen 10 is visible outside the casing thanks to an opening 14 made in back part 13.
An inside layer 17, placed between outside layer 16 and enclosure 2 of the XII tube, constitutes a shielding that assures the protection of the electron optical system from outside magnetic fields. This inside or shielding layer 17 is made of a material exhibiting a high magnetic permeability, such as, for example, soft iron, or an iron-based alloy such as, for example, "permalloy" (iron-nickel alloy), or mu-metal, etc.
It should be noted that inside layer 17 that forms a shielding from the magnetic fields is made from foils of these materials having a high magnetic permeability.
A second inside layer 18, placed between enclosure 2 of the XII tube and the first inside layer 17, constitutes a screen whose function is to absorb the x-radiation. The object of this screen 18 is to attenuate the x-radiation which leaves casing 15.
The materials that are used to make this second inside layer 18 are also generally available in the form of foils, so that the making of this second layer or screen 18 (but also of first inside layer 17) relies on the techniques of boiler making: partial stamping, working, rolling, mechanical welding. These techniques result in long and costly operations which further result in high geometric tolerances.
To produce the second inside layer or screen 18, it is common to use materials having a high atomic number, and more particularly lead which exhibits the advantage of having a cost that is not very high and the ability for greatly absorbing the x-radiation.
However, lead is a material that is difficult to use because of its great malleability or ductility. Moreover, working with lea is regulated and the personnel who perform this work are subjected to constraints of medical observation.
Another point that poses a problem in the production of XII tube casings comes from the difficulty that there is in bonding the various layers 16, 17, 18 to one another.
These layers are often bonded by gluing, which constitutes an operation that is not very compatible with industrial requirements and which results, in particular, in high tolerances on the wall thicknesses.
The absorption of the x-radiation has as its goal to attenuate the x-radiation leaving the casing, up to a value compatible with determined regulations. Since the intensities of this incident x-radiation are not equal at all points of casing 15, the absorption of the radiation must not necessarily be equal at all these points, and it is enough to impart to the wall of the casing at these various points the appropriate coefficient of absorption to obtain the desired attenuation.
As shown in FIG. 2, it is possible to define in an XII tube casing several main zones a, b, c, d, e, which extend from the front 21 of the casing on the side of input window 3 up to the back 13 of the casing on the side of output window 4. Each of these zones can require an attenuation of the x-radiation that is different from that of an adjacent zone. For example, zone b where the tube is at its largest diameter requires overall to attenuate the x-radiation less than zone c where this diameter steadily decreases.
In each of zones a to e, the thickness of screen or layer 18 that absorbs the x-rays is calculated to reduce to the determined regulation value the highest level of incident x-rays existing in each zone.
But in the same zone, there can be considerable differences in the level of x-radiation, and the thickness of layer o screen 18 is selected for the attenuation of the x-radiation having the highest intensity.
Consequently, the quantity of material constituting screen 18 is superabundant in many of its points, which results in increases in weight and cost.
Thus, for example, it is common to impart to the various zones a, b, c, d, e of screen 18, if it consists of lead foils, thicknesses respectively of about 1.7 mm, 1.2 mm, 2.3 mm, 1.2 mm and 2.7 mm, whereas for zone b, for example, the greater part of this zone could have a thickness of only 0.6 mm.